Two-dimensional silicon controlled rectifier

ABSTRACT

A two-dimensional silicon controlled rectifier (2DSCR) having the anode and cathode forming a checkerboard pattern. Such a pattern maximizes the anode to cathode contact length (the active area) within a given SCR area, i.e., effectively increasing the SCR width. Increasing the physical SCR area, increases the current handling capabilities of the SCR.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent applicationSer. No. 60/585,934, filed Jul. 7, 2004, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to a siliconcontrolled rectifier (SCR) layout within an integrated circuit (IC) and,more particularly, to an SCR layout that maximizes the width/area ratioof an SCR layout.

2. Description of the Related Art

Silicon-controlled rectifiers (SCRs) find widespread use in manyapplications where a switched resistive path is desired. For example,SCRs find particular use in electrostatic discharge (ESD) circuits ordevices that are placed between an output or input pad of an IC andground. The ESD circuits provide a high resistance path through an SCRwhen the circuit being protected is operating normally, and provides alow resistance path from the pad to ground when an ESD event occurs. TheESD event triggers the SCR into a low resistance state that shunts ESDgenerated current from the pad to ground. In this manner, the circuitbeing protected is not damaged by the ESD event.

There are many variations of ESD circuits that utilize SCRs in thismanner. Commonly assigned U.S. Pat. Nos. 6,768,616, 6,791,122, 6,850,397and 6,909,149, which are incorporated herein by reference, describe anumber of such ESD circuits.

In conventional SCRs, the amount of current that can be handled by anSCR is proportional to the width of the SCR. FIG. 1 depicts a simplifiedtop plan view of a 1-sided SCR 100. The SCR comprises an anode 102 and acathode 104 that abut one another along an active area 106. As is wellknown in the art, the SCR 100 is generally formed of a PNPN device, thedetails of which are well-known and not shown in this simplified view.The SCR 100 has a length L and a width W. When the SCR 100 is nottriggered, the SCR has a high resistive path from the anode 102 tocathode 104. Conversely, when the SCR 100 is triggered in a lowresistance state, current I flows from the anode 102 to the cathode 104along the width W of the SCR 100, i.e., along the active area 106. Thewider the SCR width W, the wider the active area 106 and the higher thecurrent handling capability of the SCR. Since the depth of the SCR 100is fixed by the IC manufacturing parameters, the current handlingability of the SCR is solely controlled by the SCR width W. In an ESDcircuit, an ESD circuit designer selects an SCR width that provides asuitable level of ESD protection against an ESD event.

To increase the current handling capability without increasing thephysical width of the SCR, a 2-sided SCR may be used. FIG. 2 depicts asimplified top plan view of a 2-sided SCR 200 comprising a first anode202, a cathode 204 and a second anode 206. The two anodes 202 and 206abut the cathode 204 along active areas 208 and 210. When the SCR isactive, current flows from both anodes 202, 206 into the cathode 204. Assuch, the effective width of the SCR is double that of the 1-sided SCR.As with the 1-sided SCR, an ESD designer controls the current handlinglevel by adjusting the physical width W of the SCR 200.

If the region of the circuit in which the SCR is formed has limitedspace for the SCR width, the designer may not be able to achieve thewidth of the SCR that is necessary for the desired current handlingcapability. The result will be a compromised design.

Therefore, there is a need in the art for increasing the currenthandling capability of an SCR without increasing the physical width ofthe SCR, i.e., increasing the current handling for a given SCR area.

SUMMARY OF THE INVENTION

The present invention is a two-dimensional silicon controlled rectifier(2DSCR) having the anode and cathode forming a checkerboard pattern.Such a pattern maximizes the anode to cathode contact length (the activearea) within a given SCR area, i.e., effectively increasing the SCRwidth. Increasing the physical SCR area, increases the current handlingcapabilities of the SCR.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a simplified top plan view of a conventional 1-sided SCR;

FIG. 2 is a simplified top plan view of a conventional 2-sided SCR;

FIG. 3 is a simplified top plan view of the two-dimensional SCR (2DSCR)of the present invention;

FIG. 4 is a top plan view of four wells of the 2DSCR of FIG. 3;

FIG. 5 depicts a graph of the width/area ratio of a 2DSCR compared to aconventional 1-sided SCR;

FIG. 6 depicts a graph of the width/area ratio of a 2DSCR compared to aconventional 2-sided SCR;

FIG. 7 depicts a top view of width/area ratio of a 2DSCR compared to aconventional 2-sided SCR, where S=2.82 um and LnLp=0.31 um;

FIG. 8 depicts a top view of width/area ratio of a 2DSCR compared to aconventional 2-sided SCR, where S=2.0 um and LnLp=0.1 um

FIG. 9 depicts a graph of the SCR width as a function of S for a 2DSCR;

FIG. 10 depicts a graph of the SCR width as a function of S for a 2DSCRusing a plurality of LnLp values;

FIG. 11 depicts a top plan view of an exemplary layout of an SCRfabricated in accordance with the present invention; and

FIG. 12 depicts a top plan view of a second exemplary layout of an SCRfabricated in accordance with the present invention.

DETAILED DESCRIPTION

The present invention is a two-dimensional silicon controlled rectifier(2DSCR) having an anode and cathode arranged in a checkerboard pattern.Such a pattern increases the effective width of the SCR and enables thecurrent handling capability to be controlled by the area of the SCR,i.e., in two-dimensions.

FIG. 3 depicts a simplified top plan view of a 2DSCR 300 comprising aplurality of anodes 302 and cathodes 304 arranged in a checkerboardpattern. Each anode 302 and cathode 304 is substantially square wherethe active edges of the anode and cathode regions are substantiallyequal in length. Such a structure provides functional uniformity.However, in some designs, an asymmetric function may be desirable suchthat rectangular or other shaped anode and cathode regions may be used.

The anodes and cathodes are positioned alternately next to each other inrows and columns to form the checkerboard pattern. In a layer or layersabove the pattern (but not shown in FIG. 3, an interconnect structure isused to interconnect all the anodes 302 and to interconnect all thecathodes 304 to create a practical SCR device. Such interconnectstructures are well-known in the art.

When not triggered, the anode-cathode junctions (i.e., the active areasbetween the anodes and cathodes) operate in a high resistance state.However, when triggered, the junctions operate in a low resistance stateand current will flow from the plurality of anodes into the plurality ofcathodes. Depending upon the position within the SCR 300 for aparticular anode or cathode, the anode or cathode will have two to fourssides that conduct current. For example, the corner elements 308 (anodeor cathode) have 2 sides that contact neighboring elements, the centerelements 310 have four sides contacting neighbors and the edge elements312 have three sides contacting neighbors. Along each of the contactedges, an active area is formed and current will flow. In the depictedembodiment, there are 45 sides (of length S) that form active areas andfacilitate current flow. Using a simple example, assuming each side is Sunits of length, the 2DSCR has 45S units of equivalent width. In aconventional 1-sided SCR, the width would be 8S units and a 2-sided SCRwould have 16S units of width. Since the present invention uses theentire area of the SCR to enhance the effective width of the SCR, theeffective width is far greater than has been achieved previously.However, in this simple example, it was assumed that current would flowalong an entire side of an anode or cathode. Because of the structure ofthe N and P wells of the SCR, a junction region that forms the activeareas consumes some of the side length S.

FIG. 4 depicts two P-type anodes 402 and 408 in an N-well 410 and 416,and two N-type cathodes 400 and 404 in a P-well 414 and 412 of the SCR300 of FIG. 3. In addition, trigger taps (not shown) may be employed toenhance the operation of the SCR and be positioned within the anode andcathode regions. Such trigger taps are disclosed in detail in commonlyassigned U.S. Pat. Nos. 6,768,616, 6,791,122, 6,850,397 and 6,909,149.

The actual SCR width must account for the distance that the anode orcathode is from the well edge. The distance from the P-type well 412/414to the N-type cathode 404/400 is referred to as Ln, and the distancefrom the N-type well 410/416 to the P-type anode 402/408 is referred toas Lp. When both distances are equal, they are concatenated to formdistance LnLp. In the simplified layout of FIG. 4, the well edges areshown as contributing to the SCR width. These distances are used to findthe actual width of the SCR. The actual element width (Saa) thatcontributes to current flow is the well edge length S minus 2LnLp, orSaa=S−2LnLp. Summing the widths Saa of all the elements that abut oneanother provides an overall effective width of the 2DSCR.

After adjusting for LnLp, the total effective width of the SCR 300 wascalculated as a function of the SCR's area to produce a graph. FIG. 5depicts a graph 500 of the area of the SCR, represented by the XY-planebounded by the X-axis and Y-axis, as well as the SCR width, representedby the Z-axis. Surface 502 represents the SCR width of a conventional2-sided SCR. In contrast, surface 504 represents the SCR width for a2DSCR of the present invention. As is clearly shown, for some minimumvalue of X and Y, the width of the 2DSCR increases much faster than theconventional SCR. As such, a much greater effective width can begenerated for a given SCR are using the 2DSCR. For this graph, S=2.82 umand LnLp=0.31 um.

To emphasize the dramatic increase in width for the 2DSCR versus aconventional SCR, FIG. 6 depicts a graph 600 of the difference 604between surfaces 502 and 504 of FIG. 5. The Z-plane is shown as surface602 to provide a reference.

FIG. 7 depicts a graph 700 of the difference between SCR widths as iflooking down into the XY-plane. In this example, S=2.82 um and LnLp=0.31um. For X and Y dimensions of the SCR area that are greater than 8 um,the 2DSCR layout provides a greater SCR width than the conventional2-sided SCR layout. In practical SCRs, the dimensions of X or Y isgenerally larger than 20 um, the 2DSCR layout is advantageous in nearlyall practical implementations of SCRs.

FIG. 8 depicts a graph 800 of the difference between SCR widths as iflooking down into the XY-plane, where in this example, S=1.0 um andLnLp=0.1 um. Note that on this smaller scale, the 2DSCR is advantageousover the conventional SCR when the X and Y dimensions of the SCR areaare greater than 2.5 um.

FIG. 9 depicts a graph 900 of SCR width as a function of S with the SCRarea being constant, e.g., 10 um×10 um. Such a graph can be used toidentify the optimal value of S given a fixed area and LnLp. In thegraph of FIG. 9, the area is 100 um² and LnLp=0.3 um. The optimal widthis 1.15 um. Using this graphical analysis, an optimal width value can befound for pairs of area and LnLp values. For example:

X (um) Y (um) Optimal S (um) 10 10 1.15 100 10 1.15 1000 10 1.15 1000100 1.2 1000 1000 1.2

FIG. 10 depicts a graph 1000 of the SCR width as a function of S, wherethe area is held constant. Each of the curves corresponds to a differentvalue of LnLp, varying from 0.1 um to 0.5 um. As the value of LnLpincreases, the value of S that optimizes the SCR width also increases.

FIG. 11 is a 2DSCR layout 1100 comprising two wells 1102, 1106 havinganodes 110, 1116 and two wells 1104, 1116 having cathodes 1112 and 1114created using a TSMC 0.13 um fabrication process. The TSMC 0.13 umprocess is a widely used, deep sub-micron IC manufacturing process. Thelayout 1100 has S=2.82 um and LnLp=0.31 um. The pattern can be expandedin both X and Y directions by adding more wells to achieve a desired SCRwidth. Also, in this layout 1100, the center of each anode and cathoderegions contain a trigger taps 1118, 1120, 1122, 1124. These triggertaps may be located in either the anode, cathode or both.

As a matter of practicality, sometimes manufacturing rules prohibitcorner-to-corner contact of shapes within a common layer, i.e., cornersof wells. To enhance manufacturability, the corners of one conductivitytype well can be altered to create an acceptable geometric shape, e.g.,a polygon. FIG. 12 depicts a 2DSCR layout 1200 comprising two wells1202, 1206 having anodes 1210, 1216 and two wells 1204, 1216 havingcathodes 1212 and 1214 that are manufacturable using a TSMC 0.13 umfabrication process. In the depicted embodiment, the N-wells areconnected at the corners (region 1218). Of course, in an alternativeembodiment, the P-wells may be connected at the corners in the samemanner. In this layout 1200, trigger taps 1220, 1222, 1224, 1226 aredepicted at the center of the anode region and the cathode region.Alternatively, the taps can be in either the anode, cathode or both. Ina further alternative, trigger taps can be added to the well cornerareas 1228. The type and position of the trigger taps are selected tosupport the application of the SCR.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A silicon controlled rectifier (SCR) comprising: a plurality ofanodes within a plurality of N-type material wells, respectively; aplurality of cathodes within a plurality of a P-type material wells,respectively; where the anodes and cathodes are arranged in an area ofan integrated circuit in a checkerboard pattern, wherein each of theanodes comprises a P-type material and each of the cathodes comprises anN-type material; an N-type material trigger tap positioned directly inthe N-type material well of each of the anodes, wherein the N-typematerial wells and the P-type material wells are substantially same insize and shape, and arranged alternately along each of two maindirections that are substantially perpendicular to each other.
 2. TheSCR of claim 1, wherein each of the anodes and cathodes in thepluralities of anodes and cathodes has a substantially square plan form.3. The SCR of claim 1, wherein each of the anodes and cathodes in theplurality of anodes and cathodes has a polygonal plan form.
 4. The SCRof claim 1, further comprising a P-type trigger tap positioned in theP-type material well of each of the cathodes.
 5. The SCR of claim 1,wherein either the P-type material wells of the cathodes or the N-typematerial wells of the anodes are connected.
 6. The SCR of claim 1,wherein a width of an active area that contributes to current flow fromthe anode to the cathode is defined by a well edge length minus Ln andLp, wherein the well edge length is a physical length of the P-typematerial well or the N-type material well, Ln is a distance from an edgeof the P-type well to an edge of the N-type cathode, and Lp is thedistance from an edge of the N-type well to an edge of the P-type anode.7. The SCR of claim 6, wherein a summation of the widths of the activeareas produces an effective width of the SCR.
 8. The SCR of claim 6,wherein the active area defines a current handling capability of theSCR.